[release-v2.0] mixclient: Introduce random message jitter

In this change, random per-peer (per-identity) and per-message jitter are
added to add additional delay to all message broadcasts.  The jitter delay
will range from 0-3s, with a random 0-2700ms of that being per-peer and
0-300ms being per-message.  The total delay is calculated as 1/10 of the stage
timeout duration, and the per-message delay is calculated is both the mean and
median of dcrd's inventory trickling delay.

While here, pair request messages are also submitted to the network with
random delays spanning over the entire epoch duration, while avoiding any PR
sends during the 30s before and after the epoch.

mixing/mixclient/blame.go

@@ -65,13 +65,13 @@ func (c *Client) blame(ctx context.Context, sesRun *sessionRun) (err error) {
 		}
 	}()
 
-	err = c.forLocalPeers(ctx, sesRun, func(p *peer) error {
+	err = c.sendLocalPeerMsgs(ctx, sesRun, true, func(p *peer) mixing.Message {
 		// Send initial secrets messages from any peers who detected
 		// misbehavior.
 		if !p.triggeredBlame {
 			return nil
 		}
-		return p.signAndSubmit(p.rs)
+		return p.rs
 	})
 	if err != nil {
 		return err
@@ -88,21 +88,23 @@ func (c *Client) blame(ctx context.Context, sesRun *sessionRun) (err error) {
 	}
 
 	// Send remaining secrets messages.
-	err = c.forLocalPeers(ctx, sesRun, func(p *peer) error {
+	err = c.sendLocalPeerMsgs(ctx, sesRun, true, func(p *peer) mixing.Message {
 		if p.triggeredBlame {
 			p.triggeredBlame = false
 			return nil
 		}
 		p.rs.SeenSecrets = rsHashes
-		return p.signAndSubmit(p.rs)
+		return p.rs
 	})
 	if err != nil {
 		return err
 	}
 
 	// Wait for all secrets, or timeout.
 	rcv.RSs = make([]*wire.MsgMixSecrets, 0, len(sesRun.prs))
-	_ = mp.Receive(ctx, rcv)
+	rcvCtx, rcvCtxCancel := context.WithTimeout(ctx, timeoutDuration)
+	_ = mp.Receive(rcvCtx, rcv)
+	rcvCtxCancel()
 	rss := rcv.RSs
 	for _, rs := range rcv.RSs {
 		if idx, ok := identityIndices[rs.Identity]; ok {